The present invention relates to a method of manufacturing a Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) device, and more particularly to a method of manufacturing a trench-type Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) device.
Nowadays, trench-type MOSFET devices are broadly used in the semiconductor industry. The difference between the trench-type MOSFET device and the traditional MOSFET device is that the gate structure of the former is formed in the trench for minimizing the area of the MOSFET device, enhancing the density of the MOSFET device and preventing the on-resistance from increasing rapidly in a short time. However, thinning the gate oxide layer for enhancing the current drive renders the gate oxide layer more easily affected by the punch-through effect.
Generally speaking, in the exemplary fabrication of a trench-type MOSFET device, a pad oxide layer, a silicon nitride layer and a mask oxide layer are sequentially formed on a semiconductor substrate. After that, the photolithography process and the etching process will be performed on the semiconductor substrate to remove portions of the mask oxide layer, the silicon nitride layer, the pad oxide layer and the semiconductor substrate to form trenches on the semiconductor substrate. Finally, the polysilicon layer is deposited on the semiconductor substrate to form a gate electrode. However, as described above, the trench-type MOSFET device with thinner gate oxide layer is easily affected by the punch-through effect. Thus, in order to reduce the punch-through effect, a trench-type MOSFET device with a bottom-oxide layer is therefore developed. The detailed description of the fabrication of the trench-type MOSFET device with a bottom-oxide layer can be found in U.S. Pat. No. 6,265,269.
In the fabrication of MOSFET devices, because of the arrangements of circuits on the semiconductor substrate, the trenches are not distributed uniformly and formed densely and loosely in different districts on the semiconductor substrate. The photoresist will be formed on the semiconductor substrate with an obvious difference in thickness. The thickness of the bottom-oxide layers of different working devices will also be non-uniform at the bottom of trenches in the subsequent steps. In another aspect, the etching process performed for forming the bottom-oxide layer also generates undesired damage in the portion of semiconductor substrate near the top of the trench. As a result, wafer acceptance test (WAT) properties of the MOSFET device will be affected, so as to influence the trench-type MOSFET.
It is therefore desirable to provide a method of manufacturing trench-type MOSFET devices with bottom-oxide layers having uniform thickness in the trenches and to avoid undesired damage on the semiconductor substrate of the trench-type MOSFET device near the top of the trenches.